Method for production of high purity silicon

ABSTRACT

The invention relates to a method for the production of high purity silicon, characterized by the following steps: a) reaction of metallic silicon with silicon tetrachloride (SiCl 4 ), hydrogen (H 2 ) and hydrochloric acid (HCl) at a temperature of 500 to 800° C. and a pressure of 25 to 40 bar to give a trichlorosilane-containing (SiHCl 3 ) feed gas stream, b) removal of impurities from the resultant trichlorosilane-containing feed gas stream by scrubbing with condensed chlorosilanes at a pressure of 25 to 40 bar and a temperature of 160 to 200° C. in a multi-stage distillation column, to give a purified trichlorosilane-containing feed gas stream and a solid-containing chlorosilane suspension and a distillative separation of the purified feed gas stream into a partial stream essentially comprising SiCl 4  and a partial stream, essentially comprising SiHCl 3 , c) disproportionation of the SiHCl 3 -containing partial stream to give SiCl 4  and SiH 4 , whereby the disproportionation is carried out in several reactive/distillative reaction zones, with a counter-current of vapour and liquid, on catalytic solids at a pressure of 500 mbar to 50 bar and SiHCl 3  is introduced into a first reaction zone, the lower boiling SiH 4 -containing disproportionation product produced there undergoes an intermediate condensation in a temperature range of −25° C. to 50° C., the non-condensing SiH 4 -containing product mixture is fed to one or more further reactive/distillative reaction zones and the lower boiling product thus generated, containing a high proportion of SiH 4  is completely or partially condensed in the head condenser and d) thermal decomposition of the SiH 4  to give high purity silicon.

The present invention relates to a method for producing high-puritysilicon by reaction of metallic silicon with silicon tetrachloride(SiCl₄), hydrogen (H₂), and hydrochloric acid (HCl), removal ofimpurities from the resultant trichlorosilane-containing (SiHCl₃) feedgas stream, disproportionation of SiHCl₃ to give SiCl₄ and silane(SiH₄), and thermal decomposition of the SiH₄ to give high-puritysilicon.

High-purity silicon is needed in the production of semiconductors andsolar cells. In this context, the required purity level of the siliconis very high. The production of high-purity silicon meeting theserequirements is performed in accordance with “Silicon for the ChemicalIndustry IV, Geiranger, Norway, Jun. 3-5, 1998, Ed.: H. A. Øye, H. M.Rong, L. Nygaard, G. Schüssler, J. Kr. Tuset, pp. 93-112” followingthree different methods:

-   -   reaction of metallic silicon with hydrochloric acid (HCl) in a        fluid bed to give SiHCl₃, purification of SiHCl₃, thermal        decomposition of the purified SiHCl₃ in the presence of H₂ on        silicon bars to give high-purity silicon. The SiCl₄-containing        reaction gas generated during the thermal decomposition of        SiHCl₃ is condensed and purified. It is a drawback of this        method that large quantities of SiCl₄ are produced as a        by-product which either reacts in a separate, technically and        energetically very expensive process step to give SiHCl₃ or has        to be utilised, for example in the production of pyrogenic        silicic acid.    -   Reaction of silicon tetrafluoride (SiF₄) with sodium-aluminium        hydride (NaAlH₄) to give SiH₄ and sodium-aluminium fluoride        (NaAlF₄), purification of the resultant SiH₄, separation of        high-purity silicon on silicon seed particles in a fluid bed,        and removal of H₂ from the generated granulated high-purity        silicon. Large quantities of NaAlF₄ are produced which have to        be utilised or sold. Another drawback of this method is that        during the separation of high-purity silicon in the fluid bed a        considerable amount of high-purity silicon powder is produced        which has to be disposed of as there is no way of economic        utilisation.    -   Reaction of metallic silicon with SiCl₄ and H₂ in a fluid bed to        give SiHCl₃, catalysed 2-stage disproportionation of SiHCl₃ to        give SiCl₄ and SiH₄, feedback of the generated SiCl₄ into the        reaction of metallic silicon with SiCl₄ and H₂, thermal        decomposition of the generated SiH₄ on silicon bars to give        high-purity silicon and feedback of the H₂ generated during this        process into the reaction of metallic silicon with SiCl₄ and H₂.

The last of the methods described above is characterised by the factthat a forced generation of large quantities of by-products is avoideddue to the utilisation of SiCl₄ generated during the process in theproduction of SiHCl₃, i.e. by reacting it with metallic silicon andhydrogen.

Embodiments of this method are described in “Studies in OrganicChemistry 49, Catalyzed Direct Reactions of Silicon, Elsevier, 1993, pp.450 through 457” and DE 3 311 650 C2 and CA-A-1 162 028. According tothese documents, the production of high-purity silicon following thismethod comprises the following steps:

1. Reaction of metallic silicon with SiCl₄ and H₂ at 400 to 600° C. anda pressure of 20.7 to 41.4 bar in a fluid bed reactor.

2. Removal of impurities such as non-reacted fine-grain silicon, metalchlorides and catalyst, if applicable, from the generatedchlorosilane-containing and hydrogen-containing reaction mixture byscrubbing the hot gas stream with condensed chlorosilanes.

3. Disposal of the solid-containing chlorosilane suspension thusgenerated.

4. Condensation of the purified reaction mixture.

5. Feedback of the hydrogen generated in step 4 to step 1.

6. Distillative separation of the purified reaction mixture to giveSiCl₄ and SiHCl₃.

7. Feedback of SiCl₄ to step 1.

8. 2-stage catalysed disproportionation of SiHCl₃ generated in step 6 togive SiH₄ and SiCl₄.

9. Feedback of SiCl₄ to step 1.

10. Distillative purification of SiH₄ generated in step 8.

11. Thermal decomposition of the purified SiH₄ while generatinghigh-purity silicon and hydrogen.

12. Utilisation of the hydrogen thus generated as thinning gas in step11 or feedback to step 1.

During the execution of step 1, the yields obtained fall significantlyshort of the yields to be expected in the light of the thermodynamicequilibrium due to the slow reaction speed. In order to accelerate thereaction, DE 3 311 650 C2 and DE 4 104 422 A1 suggest the use of coppercatalysts. The copper catalysts are usually fed into the fluid bedtogether with the ground silicon or separately. The drawback of thismethod is that in the fluid bed a portion of the catalyst is directlydischarged from the fluid bed together with the gaseous reactants and/orthe reaction products due to insufficient adhesion to the siliconparticles and thus is no longer available for the reaction. This causesa considerable need of copper catalyst adversely affecting the economicefficiency of this method with a view to the normally high price ofcopper catalysts.

During the reaction of metallic silicon with SiCl₄ and hydrogen,chlorides such as aluminium chloride (AlCl₃), iron chloride (FeCl₂) andcalcium chloride (CaCl₂) are generated in addition to the desiredchlorosilanes. Most of these metal chlorides are removed as crystallisedsolid particles in step 2 by scrubbing the hot gas stream with condensedchlorosilanes. Due to the high sublimation pressure, crystallised AlCl₃may, however, be distributed via the gas chamber in the entire scrubbingfacility. This causes deposition of AlCl₃ on the scrubbing facility andits internal components so that periodic shutdowns and cleaning measuresof the scrubbing facility are required.

The removal of AlCl₃ by distilling the chlorosilanes can be performed atlowered temperatures in a vacuum. Under these conditions, the vapourpressure of the solid aluminium chloride is so low that its share in thegaseous phase may fall below the solubility limit so that the problem ofdeposition of solid aluminium chloride in the facility is avoided.However, it is not possible to lower the AlCl₃ portion below the shareof its vapour pressure in the entire vapour pressure so that on the onehand the separation of AlCl₃ is possible only to a limited degree whileon the other hand due to this fact the problem of aluminium chloridedeposition during another distillation process occurs anew, although atlesser quantities. The method described in DE 2 161 641 A1 offers apartial solution of this problem wherein the gas stream leaving thesynthesis reactor is cooled in two stages by firstly feeding SiCl₄ toreach 250 to 300° C. and secondly by using a Venturi scrubber with moreSiCl₄ to reach approx. 53° C., wherein iron chloride and aluminiumchloride carried by the chlorosilanes are deposited and remain in thecondensation product while the gas stream containing the major portionof the chlorosilanes is again scrubbed with condensed chlorosilanes sothat it can be condensed thereafter. The remaining content of AlCl₃ isthen determined by the vapour pressure of AlCl₃ at approx. 53° C. Duringthe further purification of the chlorosilanes, the aforementionedproblem of AlCl₃ concentration in the sump occurs again, causing theformation of a solid phase of AlCl₃ and its undesired sublimation. Quitesimilarly, the method described in DE 2 623 290 A1 offers only a partialsolution wherein the gas stream containing the chlorosilanes and AlCl₃is cooled down to 60 to 80° C. with the purpose of letting the solidAlCl₃ deposit and separating it.

The removal of the metallic silicon and metallic chloride-containingchlorosilane suspension generated in step 2 is performed in accordancewith DE 3 709 577 A1 by a specific distillative separation of thechlorosilanes from the solid particles wherein a large portion of thechlorosilanes can be recovered. The remaining solid andchlorosilane-containing distillation sump cannot be utilised and thushas to be disposed of as described for example in U.S. Pat. No.4,690,810. This process has an adverse effect on the economy of thismethod. Another drawback is the fact that disturbing impurities are,together with the recovered chlorosilanes, fed back into the process ofproducing high-purity silicon which may cause an undesired concentrationof these impurities and hence adversely affect the process.

During the steps 4, 6, 8 and 10, silane and chlorosilane-containingwaste streams are generated which, as a rule, are disposed of byscrubbing with solutions of alkali compounds or, for example, bycombustion. Several variants of the method for disposing of the wastestreams are for example described in U.S. Pat. No. 4,519,999. In thesemethods, the reactive silicon compounds are made harmless by convertingthem into silicates or silicic acids. However, the drawback of thismethod is that the actually valuable waste stream components such asSiH₄, SiH₂Cl₂, SiHCl₃ and SiCl₄ are converted into less valuableproducts.

The 2-stage catalysed disproportionation of SiHCl₃ to give SiH₄ andSiCl₄ performed in step 8 requires very expensive equipment and power.According to DE 2 507 864 A1, an improved variant of the method is tohave the disproportionation of SiHCl₃ take place in a single stepfollowing the principle of reactive distillation. There is, however, agrave disadvantage in this method described in DE 2 507 864 A1, namely,that the amount of energy used for the separation of the silanes orchlorosilanes has to be completely discharged on a very low temperaturelevel of less than −50° C. to −120° C. corresponding to the condensationtemperatures. It is known that heat discharge on a low temperature levelis expensive and implies high power consumption.

The thermal decomposition of purified SiH₄ of step 11 may be performedfollowing three different methods according to DE 3 311 650 C2:

A. Semi-continuous deposition of high-purity silicon on silicon bars,known as Siemens method.

B. Continuous decomposition of SiH₄ in a gaseous phase reactor includingthe generation of powder-like high-purity silicon.

C. Continuous decomposition of SiH₄ on silicon seed particles in a fluidbed including the generation of granulated high-purity silicon.

The method variant A requires very expensive equipment and, due to theinevitable semi-continuous operation, regular plant shutdowns withexpensive cleaning work. In the method variant B, high-purity silicon isgenerated as a fine powder which cannot be directly utilised, so thatthis powder has to be compressed and molten in expensive subsequentsteps. Further, this high-purity silicon powder is easily contaminateddue to its very large specific surface and therefore can normally not beutilised in the fields of photovoltaics or semiconductor technology.

In the method variant C, however, granulated high purity-silicon iscontinuously produced which can easily be processed further. Variants ofthis method are described, for example, in U.S. Pat. No. 3,012,861 andU.S. Pat. No. 5,798,137. According to these documents, the drawbacks ofthe method variant C are that a considerable portion of the SiH₄ used isdecomposed to powder-like high-purity silicon. In order to lessen thepowder production, SiH₄ decomposition may be performed by addinghydrogen at nearly ambient pressure.

The high-purity silicon powder is, as described above, easilycontaminated due to its very large specific surface when it is furtherprocessed in subsequent steps and therefore cannot be utilised in thefields of photovoltaics or semiconductor technology, either.

The drawbacks mentioned above are the reason why the production ofhigh-purity silicon is still very expensive so that in particular theeconomic profitability and with it the growth of photovoltaicapplications using high-purity silicon are considerably affected.

It is therefore the object of the present invention to provide a methodfor producing high-purity silicon which does not imply the drawbacksmentioned above and allows a cost-effective production.

The present invention relates to the production of high-purity silicon,characterised by the following steps:

-   a) reaction of metallic silicon with silicon tetrachloride (SiCl₄),    hydrogen (H₂) and hydrochloric acid (HCl) at a temperature of 500 to    800° C. and a pressure of 25 to 40 bar to give a    trichlorosilane-containing (SiHCl₃) feed gas stream,-   b) removal of impurities from the resultant    trichlorosilane-containing feed gas stream by scrubbing with    condensed chlorosilanes at a pressure of 25 to 40 bar and a    temperature from 160 to 220° C. in a multi-stage distillation    column, to give a purified trichlorosilane-containing feed gas    stream and a solid-containing chlorosilane suspension, and    distillative separation of the purified feed gas stream into a    partial stream essentially consisting of SiCl₄ and a partial stream    essentially consisting of SiHCl₃,-   c) disproportionation of the SiHCl₃-containing partial stream to    give SiCl₄ and SiH₄, whereby disproportionation is carried out in    several reactive/distillative reaction zones, with a counter-current    of vapour and liquid, on catalytically active solids at a pressure    of 500 mbar to 50 bar and SiHCl₃ is introduced into a first reaction    zone, the lower-boiling SiH₄-containing disproportionation product    produced there undergoes an intermediate condensation in a    temperature range of −25 to 50° C., the non-condensed    SiH₄-containing product mixture is fed to one or more further    reactive/distillative reaction zones and the lower boiling point    product thus generated, containing a high proportion of SiH₄ is    completely or partially condensed in the top condenser, and-   d) thermal decomposition of SiH₄ to give high-purity silicon.

Here, high-purity silicon means silicon having a purity high enough tobe suitable in the field of photovoltaics. This requires all metalconcentrations to remain below 0.1 ppm, carbon content below 1 ppm,oxygen below 5 ppm, phosphorus below 0.1 ppm, and boron below 0.05 ppm.

Metallic silicon means silicon which may contain up to approximately 3wt. % iron, 0.75 wt. % aluminium, 0.5 wt. % calcium, and otherimpurities as being usually found in silicon and which was preferablyobtained by carbo-thermic reduction of silicon dioxide.

It was found that the execution of the method according to the presentinvention brings about a number of advantages allowing a significantlymore cost-effective production of high-purity silicon.

In the reaction of metallic silicon with SiCl₄, H₂ and HCl at atemperature of 500 to 800° C., preferably 550 to 650° C., and a pressureof 25 to 40 bar, preferably 30 to 38 bar, high space/time yields ofSiHCl₃ are obtained.

The addition of hydrochloric acid, preferably at an amount of 0.05 to 10wt. %, most preferably 1 to 3 wt. %, each being relative to the amountof SiCl₄ added as additional reactant, causes an unexpected accelerationof the reaction which finally leads to the fact that very large SiHCl₃yields, i.e. high reaction levels of the SiCl₄ used near thethermodynamic equilibrium, and at the same time high total yields, i.e.large-scale utilisation of the metallic silicon used, are achieved.

Hydrochloric acid is preferably used in its water-free form ashydrochloric gas.

Hydrochloric acid can, for example, be separately fed into the reactorin which the reaction to produce chlorosilane is to be performed.However, it is also possible to feed hydrochloric acid together with thegaseous and/or evaporable initial substances to be fed into the reactor,namely hydrogen and/or silicon tetrachloride.

The selection of the reactor in which the reaction according to thepresent invention is to be performed is not critical as long as thereactor ensures enough stability with respect to the reaction conditionsand allows the contact of the initial substances. Applicable reactorsare, for example, a solid bed reactor, a rotating tube furnace or afluid bed reactor. Preferably the reaction is performed in a fluid bedreactor.

The material of the reactor has to withstand the reaction conditionsmentioned with respect to SiHCl₃ synthesis. The requirements regardingthe durability of the structural reactor materials are applicable aswell to possible up- or downstream facility components such as cyclonesor heat exchangers. These requirements are met, for example, bynickel-based alloys.

The use of catalysts allows an additional acceleration of the reactionof metallic silicon with SiCl₄, H₂ and HCl. Especially suitablecatalysts are copper, iron, copper or iron compounds or mixturesthereof.

Surprisingly, it became evident that the catalysts were particularlyeffective when the metallic silicon was grounded and thoroughly mixedwith the catalysts prior to the reaction.

In the method according to the present invention, the reaction to givetrichlorosilane (step a)) is preferably performed in the presence of acatalyst, wherein the metallic silicon is thoroughly mixed with thecatalyst prior to the reaction.

Preferably, silicon having mean grain diameters between 10 and 1000 μm,preferably 100 to 600 μm, is used. The mean grain diameter is determinedas the numerical average of the values obtained in a sieving analysis ofthe silicon.

For mixing the catalyst and the silicon, devices are preferably usedwhich ensure very thorough mixing. Mixers having rotating mix tools aremost suitable for this purpose. Such mixers are described, for example,in “Ullmann's Encyclopedia of Industrial Chemistry, Volume B2, UnitOperations 1, pp. 27-1 to 27-16, VCH Verlagsgesellschaft, Weinheim”.Most preferably, ploughshare mixers are used.

During the thorough mixing, the catalyst may be crushed further so as toensure during the mixing process a very good distribution and a verygood adhesion of the catalyst to the silicon surface. Thus, evencatalysts which are not available as fine particles and/or cannot becrushed to the fineness required may be used.

In case of insufficient mixing, a major portion of the catalyst is, dueto its low adhesion to the silicon particles, directly discharged fromthe fluid bed together with the gaseous reactants and/or products andthus is no longer available for the reaction. This raises the need forcatalyst and adversely affects the profitability of the method. This isprevented by thoroughly mixing the silicon and the catalyst.

The time period for mixing the silicon and the catalyst is preferably 1to 60 minutes. Longer mixing times are normally unnecessary. Mixingtimes of 5 to 20 minutes are particularly preferred.

The thorough mixing of the catalyst and the silicon may be performed,for example, in an inert atmosphere or in the presence of hydrogen orother gases having a reducing effect such as carbon monoxide. Amongother effects, this prevents the formation of an oxidic layer on theindividual silicon particles. Such a layer prevents the direct contactbetween the catalyst and the silicon and thus impairs the catalysedreaction of the latter with silicon tetrachloride, hydrogen and, ifapplicable, hydrochloric acid to give trichlorosilane.

An inert atmosphere may for example be generated by adding an inert gasduring the mixing process. Suitable inert gases include for examplenitrogen and/or argon.

The mixing of silicon and catalyst is preferably performed in thepresence of hydrogen.

In principle, any catalyst known for the reaction of silicon withsilicon tetrachloride, hydrogen and, if applicable, hydrochloric acidmay be used.

Particularly suitable catalysts are copper catalysts and iron catalysts.Examples include copper oxide catalysts (e.g. Cuprokat® of Messrs.Norddeutsche Affinerie), copper chloride (CuCl, CuCl₂), copper metal,iron oxides (e.g. Fe₂O₃, Fe₃O₄), iron chlorides (FeCl₂, FeCl₃) andmixtures thereof.

Preferred catalysts are copper oxide catalysts and iron oxide catalysts.

Especially during the use of copper oxide catalysts and iron oxidecatalysts, it has proven to be advantageous to perform the mixing withsilicon at a temperature of 100 to 400° C., preferably 130 to 350° C.When doing so, remaining moisture adhering to the catalysts are removedwhich adversely affects the reaction of silicon with SiCl₄, H₂ and, ifapplicable, HCl. Moreover, this approach ensures a better adhesion ofthe catalyst to the silicon surface thus largely avoiding loss ofcatalyst in the fluid bed.

It is also possible to use mixtures of copper and/or iron catalysts withother catalytically active components. Such catalytically activecomponents include, for example, metal halogenides such as chlorides,bromides or iodides of aluminium, vanadium or antimony.

The quantity of catalyst used, calculated as metal, is preferably 0.5 to10 wt. %, most preferably 1 to 5 wt. % relative to the quantity ofsilicon used.

As an alternative, the method according to the present invention allowsin the reaction to give trichlorosilane (step a)) also metallic siliconhaving an iron content of 1 to 10 wt. %, preferably 1 to 5 wt. % to beused, wherein the iron is mostly homogeneously distributed in themetallic silicon, preferably in the form of a silicide.

Silicon containing homogeneously distributed iron may for example beproduced by melting a mixture of silicon and the desired quantity ofiron or by adding a desired quantity of iron to a silicon melt, followedby rapid cooling of the melt. Preferably, the addition of the desiredquantity of iron is performed as early as during the production of thesilicon.

The rapid cooling of the melt may be performed, for example, by jettingthe melt in air or by water granulation.

The preferred method for rapid cooling of a silicon melt and thus forproducing usable silicon is water granulation. During the watergranulation, liquid silicon is fed into water. This causes the siliconto cool down at extreme speed. Depending upon the process parameters, itis possible for example to obtain silicon pellets. Water granulation ofsilicon is known for example from EP 522 844 A2.

Then the iron is present in the silicon in fine particles beinghomogeneously distributed.

The mol ratio of hydrogen and silicon tetrachloride may be, for example,0.25:1 to 4:1 in step a) of the method according to the presentinvention. The preferred mol ratio is 0.6:1 to 2:1.

According to the present invention, the trichlorosilane-containing feedgas stream generated in the reaction of metallic silicon with SiCl₄, H₂and HCl (step a)) is purified by scrubbing with condensed chlorosilanesat a pressure of 25 to 40 bar, preferably 35 to 40 bar, and atemperature of 160 to 220° C., preferably 190 to 200° C., in amulti-stage distillation column (step b)).

Suitable condensed chlorosilanes include, for example, a condensed gasstream comprising trichlorosilane and silicon tetrachloride at a molratio of approximately 1:3 to 1:20.

Surprisingly, it became evident that, when the aforementionedtemperature and pressure ranges are observed, the silicon powderremnants and metal chlorides, especially AlCl₃, which may be containedin the trichlorosilane-containing feed gas stream are completelyseparated from the feed gas stream and can be removed with the condensedchlorosilanes from the scrubbing column as solids or as dissolved metalchloride (e.g. AlCl₃).

The problems described above which are caused by sublimation of AlCl₃ inthe scrubbing column are surely avoided in the approach according to thepresent invention. This ensures a faultless long-term operation of thescrubbing column and thus the entire process. Any substances stillpresent having a higher boiling point such as disilanes, polysilanes,siloxanes and hydrocarbons are removed from the feed gas stream togetherwith the condensed chlorosilanes.

A chlorosilane suspension is produced which can then be relaxed andcooled down wherein dissolved metal chlorides, especially AlCl₃, fallout almost completely except a few ppm.

Following relaxation and cooling, solids are preferably removed from thechlorosilane suspension by filtration. The solid-free chlorosilanes maybe transferred to utilisation while treating the separated solids withalkali compounds.

The filtration of the chlorosilane suspension is preferably performedusing sinter metal filter substances. Such filters are known anddescribed, for example, in “Ullmanns Encyklopädie der technischenChemie, 4^(th) edition, Vol. 19, p. 573, Verlag Chemie, 1980”.

The solid-free filtrate is an extremely suitable raw material for theproduction of pyrogenic silicic acid. Further processing, for example,by distillation, is not required. The solids produced during filtrationmay be made inert in a known manner using alkali compounds such as sodalye, Na₂CO₃, NaHCO₃ and CaO and used after inertisation as raw materialfor cement production.

In an advantageous variant of the method according to the presentinvention, the trichlorosilane-containing feed gas is made free from anyexisting powder-like solids by gas filtration prior to scrubbing withcondensed chlorosilanes. This can for example be performed in cyclones,wherein several cyclones connected in series and/or one or moremulti-cyclones can be used to achieve high separation levels. As analternative, hot-gas filters with sinter metal or ceramic candles orcombinations of cyclones and hot-gas filters may be used. This approachhas the advantage that the subsequent feed gas scrubbing issignificantly facilitated while a silicon metal-containing solid isobtained which, due to its high content of silicon, can be transferredto utilisation in metallurgical processes such as the production of ironalloys. For this purpose, the silicon metal and metalchloride-containing solid may for example react with alkali compoundssuch as soda lye, Na₂CO₃, NaHCO₃ and CaO and water, be filtrated andwashed with water to remove chloride and then be dried, if required.

The now purified feed gas stream is condensed in a known manner andseparated by preferably multi-stage distillation into a partial streammainly consisting of SiCl₄ and a partial stream mainly consisting ofSiHCl₃.

Preferably, the partial stream mainly containing SiCl₄ is fed back intothe reaction of metallic silicon with silicon tetrachloride, H₂ and HCl(step a)).

Distillation may be performed at a pressure of 1 to 40 bar. Preferablydistillation is performed at a pressure of less than 10 bar in order toachieve a good separation of SiCl₄ and SiHCl₃ with a minimum ofdistillation stages.

According to the present invention, the SiHCl₃-containing partial streamis fed to a subsequent disproportionation. It has been proven asadvantageous to remove most of the components which have a lower boilingpoint than SiHCl₃ from this partial stream in a multi-stage distillationprocess. This distillation may also be performed at a pressure of 1 to40 bar. Preferably, the distillation is performed at a pressure of lessthan 10 bar in order to achieve a good separation of compounds having alower boiling point from SiHCl₃ with a minimum of distillation stages.

In another advantageous variant, the purified SiHCl₃ is subsequentlymade free from anhydric acids such as halogenides and hydrides usingcaustic solids. Examples of anhydric acids include BCl₃, BH₃, PCl₃, HCl.The advantage is that the efficiency of the subsequent catalyseddisproportionation is not adversely affected so that a long-termfaultless operation of the disproportionation process is ensured. Thecaustic solids used may be identical with the disproportionationcatalysts used in the following step.

The contact with caustic solids may be performed, for example, in asolid bed reactor. The process is preferably performed at a pressure of1 to 50 bar, most preferably 1 to 10 bar. The temperatures may forexample be in the range from 30 to 180° C., preferably 50 to 110° C. Thetemperature to be set depends upon the stability range of the causticsolids used. In order to ensure continuous operation, two or morereactors provided with caustic solids can be connected in parallel. Itis possible to regularly switch over to a reactor filled with freshsolids to ensure a complete removal of the aforementioned impuritieswhile the consumed solids are exchanged and regenerated, if required.Similarly, a reactor can be operated as several reactors connected inseries.

The disproportionation of the purified, if required,trichlorosilane-containing partial stream (step c)) is most preferablyperformed in a column at a pressure of 1 to 10 bar, wherein said columncomprises at least two reactive/distillative reaction zones.

Disproportionation takes place on catalytically active solids,preferably in catalyst beds each comprising a bulk solid layerconsisting of solid pieces working as catalyst solids, wherein thedisproportionation products are able to flow through this layer. In thereaction zone, the bulk solid layer may be replaced by packed catalystbodies.

Suitable catalytically active solids are known and described, forexample, in DE 2 507 864 A1. Such suitable solids are, for example,solids which carry amino or alkyleneamino groups on a polystyrenestructure meshed with divinyl benzene. Amino or alkyleneamino groupsinclude for example: dimethylamino, diethylamino, ethylmethylamino,di-n-propylamino, di-iso-propylamino, di-2-chloroethylamino,di-2-chloropropylamino groups as well as the similarly substituteddialkylaminomethylene groups and the corresponding hydrochlorides or thetrialkylammonia groups derived therefrom by methylisation, ethylisation,propylisation, butylisation, hydroxyethylisation or benzylisation withchloride as counter-ion. Of course, in the case of quaternary ammoniasalts or protonised ammonia salts, catalytically active solids havingother anions, e.g. hydroxyde, sulphate, hydrogen sulphate, bicarbonateand the like, may be fed into the method according to the presentinvention, although a gradual conversion into the chloride form isinevitable under the reaction conditions, which applies to organichydroxy groups as well. Therefore, ammonia salts containing chloride ascounter-ion are preferred.

Other suitable catalytically active solids include, for example, solidsconsisting of a polyacrylic acid structure, especially a polyacrylamidestructure, which has bonded trialkylbenzyl ammonia, for example, via analkyl group.

Another suitable group of catalytically active solids includes, forexample, solids having bonded sulphonate groups to a polystyrenestructure, meshed with divinyl benzene, being confronted by tertiary orquaternary ammonia groups as kations.

As a rule, macroporous or mesoporous exchanger resins are more suitablethan gel resins.

The trichlorosilane-containing partial stream of step b) is fed to thereaction column through an inlet opening into the column at anappropriate point. Such an appropriate point is, for example, a point atwhich the column has an inner temperature corresponding to the boilingpoint of trichlorosilane at the existing pressure. In the reactionzones, a SiH₄-containing, vapour-like product mixture ascending in thereaction zone and a SiCl₄-containing liquid mixture descending in thereaction zone are formed by disproportionation of SiHCl₃.

The SiCl₄-containing liquid flowing out of the reaction zone is fedinside the reaction column into a distillative depression unit beneaththe reactive/distillative zone from which unit silicon tetrachlorideSiCl₄ may flow off as sump product.

For the SiH₄-containing product mixture ascending in the reaction zone,an intermediate condenser is provided above the reaction zone in whichcondenser the concentration of SiH₄ in the product mixture is increasedby partial condensation of components having a higher boiling point at atemperature between −25° C. and 50° C., preferably between −5° C. and40° C. The product components having a lower boiling point which werenot condensed in the intermediate condenser are supplied for a furtherconcentration increase to a second reactive/distillative reaction zonedownstream the intermediate condenser in the flow direction of theascending product components and then to a booster unit.

Preferably, disproportionation is performed so that several intermediatecondensation processes take place in the reaction zones as a whole ondifferent temperature levels ranging from −25° C. to 50° C.

The use of three or more reactive/distillative reaction zones and two ormore intermediate condensers allows the discharge of the intermediatecondensation heat at different temperature levels with low drivingtemperature differences with advantageously low power consumption.

The booster unit may be arranged inside or outside the reaction column.The product mixture leaving the booster unit is finally supplied to atop condenser where it is deposited and discharged. A portion of theproduct mixture may be fed back to the top of the reaction column.

Impurities depositing at different temperature levels in the reactioncolumn may be taken out of the column via lateral removal points.

In order to further lower the condensation energy to be discharged at avery low temperature, the feedback quantity can be decreased and a topproduct be generated having a lesser silane purity of between 25% and90%. This top product may then be separated to be further purified in adownstream separation column wherein an equal or preferably higherpressure than in the reaction column, preferably 15 bar to 100 bar, isset so that the separation column operates at higher temperatures thanthe reaction column with respect to an equal composition. The sumpproduct of the separation column may, depending upon the selectedoperating conditions, contain large quantities of trichlorosilane,dichlorosilane and monochlorosilane. The sump product may entirely orpartly be fed back into the reaction column. Impurities may, ifrequired, be removed from the system by sluicing out a partial stream.

Preferably, SiCl₄ obtained during disproportionation is fed back intothe reaction of silicon with SiCl₄, H₂ and HCl (step a)).

According to the present invention, SiH₄ obtained duringdisproportionation is thermally decomposed (step d)).

It is possible to subject SiH₄ obtained during disproportionation to adistillative purification prior to its thermal decomposition.

In a particularly preferred embodiment of the method according to thepresent invention, the thermal decomposition of SiH₄ which has beenpurified by distillation, if required, is performed on high-puritysilicon seed particles in a fluid bed at a pressure of 100 to 900 mbar.

Thermal decomposition is preferably performed at pressures from 200 to800 mbar. The pressure range between 300 and 700 mbar, most preferablybetween 400 and 600 mbar, is particularly preferred. All specifiedpressure values are absolute pressure values. The aforementionedpressure means the pressure existing behind the fluid bed as viewed inthe flow direction of the gas stream supplied.

In the thermal decomposition of silane, it is possible to add up to 50vol. % of a silicon-free gas with relation to the entire gas supplied.Preferably, the added quantity of silicon-free gas is 0 to 40 vol. %,most preferably 0 to 20 vol. %. The addition of silicon-free gas reducesthe formation of silicon powder.

Suitable silicon-free gases include, for example, the rare gases,nitrogen and hydrogen, wherein the silicon-free gases may be usedindividually or in any combination thereof. Nitrogen and hydrogen arepreferred, with hydrogen being preferred most.

The advantageous temperature range for the thermal decomposition ofsilane is between 500° C. and 1400° C. A decomposition temperature of600° C. to 1000° C. is preferred, with 620° C. to 700° C. beingpreferred most.

The high-purity silicon seed particles may be fed into the reactionchamber of a fluid bed reactor. The high-purity silicon seed particlesmay be fed from outside intermittently or continuously. However,particles being generated in the reaction chamber may be used ashigh-purity silicon seed particles as well. The high-purity seedparticles form a solid bed through which the supplied gas is blown in.The blow-in speed of the supplied gas is set so that the solid bed isfluidised and a fluid bed is formed. The relevant approach as such isknown to a person skilled in the art. The blow-in speed of the suppliedgas has to be at least equal to the loosening speed. Loosening speedmeans the speed at which a gas flows through a particle bed and belowwhich the solid bed is retained, i.e. below which the bed particlesremain mostly immobile. Above this speed, fluidisation of the bedstarts, i.e. the bed particles move, and initial bubbles are formed.

In this embodiment, the blow-in speed of the supplied gas is one to tentimes, preferably one-and-a-half to seven times, the loosening speed.

The high-purity seed particles being advantageously used have diametersbetween 50 and 5000 μm.

The high-purity seed particles may, for example, be generated bycrushing the granulated high-purity silicon generated during thermaldecomposition of SiH₄ in the fluid bed. Usual crushing methods such asgrinding imply the risk that the high-purity silicon seed particles arecontaminated during the crushing process.

Therefore, the production of the high-purity silicon seed particles ispreferably performed by their generation in the decomposition reactoritself, separation of a fraction of appropriate particle size byprocess-internal inspection and their feedback into the reactor.

The hydrogen generated during thermal decomposition of silane ispreferably fed back into the reaction of silicon with SiCl₄, H₂ and HCl(step a)).

In another preferred variant, the high-purity silicon powder generatedas a by-product during thermal decomposition of SiH₄ is, following itsseparation from the granulated high-purity silicon, in a separateprocess step heated up my microwave irradiation at wavelengths between0.5 kHz and 300 GHz to a temperature of at least 300° C. andagglomerated. By doing so, a product is obtained which, without anyfurther processing steps such as condensation and crushing, may forexample be introduced into the melting process for the production ofsilicon wafers for solar cells.

Another preferred embodiment of the method according to the presentinvention suggests that SiH₄ and SiH₂Cl₂-containing waste streams of thevarious distillation processes are collected to react with SiCl₄ to givea SiHCl₃-containing reaction mixture from which SiHCl₃ is obtained bydistillation. It is advantageous to use in this reaction liquiddisproportionation catalysts having a boiling point above the boilingpoint of SiCl₄. Suitable disproportionation catalysts include, forexample, trialkylamines and aryldialkylamines.

If desired, SiHCl₃ may be used in the disproportionation process (stepc) following further purification. Thus, the yield and the profitabilityof the entire process are improved because there is no need for adisposal of the aforementioned waste streams which causes a loss ofsilicon compounds.

The high-purity silicon obtained according to the present invention may,due to its high purity level, be easily utilised as raw material for theproduction of semiconductors and solar cells.

The method according to the present invention allows a verycost-effective production of high-purity silicon due to the utilisationof the waste streams and by-products becoming possible according to thepresent invention, higher yields from the SiHCl₃ synthesis, and theconsiderable lower overall power need.

1. A method for producing high-purity silicon, characterized by thefollowing steps: a) reaction of metallic silicon with silicontetrachloride (SiCl₄), hydrogen (H₂ and hydrochloric acid (HCl) at atemperature of 500 to 800° C. and a pressure of 25 to 40 bar to give atrichlorosilane-containing (SiHCl₃) feed gas stream, b) removal ofimpurities from the resultant of trichlorosilane containing feed gasstream by scrubbing with condensed chlorosilanes at a pressure of 25 to40 bar and a temperature from 160 to 220° C. in a multi-stagedistillation column, to give a purified trchlorosilane containing feedgas stream and a solid containing chlorosilane suspension, anddistillative separation of the purified feed gas stream into a partialstream essentially comprising SiCl₄ and a partial stream essentiallycomprising SiHCl₃, c) disproportionation of the SiHCl₃-containingpartial stream to give SiCl₄ and SiH₄, whereby the disproportionation iscarried out in several reactive/distillative reaction zones, with acounter-current of vapor and liquid, on catalytically active solids at apressure of 500 mbar to 50 bar and SiHCl₃, is introduced into a firstreaction zone, the lower-boiling SiH₄-containing disproportionationproduct produced there undergoes an intermediate condensation in atemperature range of −25 to 50° C., the non-condensed SiH₄-containingproduct mixture is fed to one or more further reactive/distillativereaction zones and the lower boiling point product thus generated,containing a high proportion of SiH₄ is completely or partiallycondensed in the top condenser, and d) thermal decomposition of the SiH₄to give high-purity silicon.
 2. A method according to claim 1,characterized in that step a) is performed in a fluid bed reactor.
 3. Amethod according to claim 1, characterized in that hydrochloric acid isintroduced at a quantity of 0.05 to 10 wt. % relative to the weight ofthe supplied SiCl₄.
 4. A method according to claim 1, characterized inthat step a) is performed in the presence of a catalyst wherein saidcatalyst and the metallic silicon are thoroughly mixed prior to beingfed into a reactor.
 5. A method according to claim 4, characterized inthat at least one of copper, iron, copper compounds, iron compounds andmixtures thereof are used as catalyst.
 6. A method according to claim 1,characterized in that metallic silicon having an iron content of 0.5 to10 wt. % is used, wherein iron is mostly homogeneously distributed amongthe metallic silicon.
 7. A method according to claim 1, characterized inthat solid components, if any, of the trichlorosilane-containing feedgas stream are separated prior to scrubbing with condensedchlorosilanes.
 8. A method according to claim 1, characterized in thatthe solid-containing chlorosilane suspension generated in step b) ismade free of solids by filtration, the solid-free chlorosilanes aretransferred to further utilization, and the solids are treated withalkali compounds.
 9. A method according to claim 3, characterized inthat filtration is performed using sinter metal filter materials.
 10. Amethod according to claim 1, characterized in that the SiCl₄-containingpartial stream obtained in step b) is fed back into the reaction ofmetallic silicon with SiCl₄, H₂ and HCl (step a)).
 11. A methodaccording to claim 1, characterized in that the partial streamconsisting mostly of trichlorosilane obtained in step b) is made mostlyfree from components having a lower boiling point than SiHCl₃ prior todisproportionation.
 12. A method according to claim 1, characterized inthat the partial stream consisting mostly of trichlorosilane obtained instep b) is brought into contact with caustic solids prior todisproportionation for the purpose of removing anhydric acids such ashalogenides and hydrides.
 13. A method according to claim 1,characterized in that disproportionation of trichlorosilane (step c)) isperformed at a pressure between 1 and 10 bar.
 14. A method according toclaim 1, characterized in that in step c) several intermediatecondensation processes at different temperature levels in the range from−25° C. to 50° C. are performed in the reaction zones as a whole.
 15. Amethod according to claim 1, characterized in that SiCl₄ obtained instep c) is fed back into the reaction of metallic silicon with SiCl₄, H₂and HCl (step a)).
 16. A method according to claim 1, characterized inthat SiH₄ generated in step c) is subjected to a distillativepurification prior to its thermal decomposition.
 17. A method accordingto claim 1, characterized in that silane and dichlorosilane-containingwaste streams of the distillative steps are collected to react withSiCl₄ to give a trichlorosilane-containing reaction mixture, withobtaining SiHCl₃ by distillation from said reaction mixture.
 18. Amethod according to claim 17, characterized in that in the reaction ofthe silane and dichlorosilane-containing waste streams with SiCl₄ liquiddisproportionation catalysts having a boiling point above the boilingpoint of SiCl₄ are used.
 19. A method according to claim 1,characterized in that the thermal decomposition of SiH₄ (step d)) onhigh-purity silicon seed particles is performed in a fluid bed at apressure of 100 to 900 mbar.
 20. A method according to claim 1,characterized in that in the thermal decomposition of SiH₄ up to 50 vol.% of a silicon-free gas, relative to the overall quantity of thesupplied gas, is added in addition to the SiH₄ from step c).
 21. Amethod according to claim 20, characterized in that hydrogen is used assilicon-free gas.
 22. A method according to claim 1, characterized inthat H₂ generated in step d) is fed back into the reaction of metallicsilicon with SiCl₄, H₂ and HCl (step a)).
 23. A method according toclaim 1, characterized in that in the thermal decomposition of purifiedSiH₄ (step d)) high-purity silicon powder is generated as by-productwhich is heated up to a temperature of at least 300° C. and agglomeratedby means of microwave irradiation in a wavelength range between 0.5 kHzand 300 GHz.